Embodiments presented herein relate generally to ultrasound imaging systems, and more particularly to nondestructive evaluation using ultrasound imaging.
Ultrasound imaging is widely used for nondestructive testing of inaccessible portions of installed equipment. In nondestructive ultrasound testing, short ultrasonic pulses with frequencies between 0.1 and 15 MHz are launched into materials to detect internal flaws or to characterize materials. Nondestructive ultrasound testing is also commonly used to determine the thickness of the test object, for instance, to monitor pipework corrosion. Nondestructive ultrasound testing is often performed on steel and other metals and alloys. In some applications, nondestructive ultrasound testing may also be used on concrete, wood and composites.
In nondestructive ultrasound testing, uniform focus in the image is desirable. Uniform focus allows all points in the image to be evaluated for potential defects at the same time without adjustment of the nondestructive ultrasound tester. In medical imaging, the standard approach to achieving uniform focus is to use multiple transmit focal zones with dynamic receive focusing. However, such an approach typically does not yield sharp focus to enable multiple focal zones in the standard 16-element nondestructive ultrasound tester because of the large depth-of-focus of such a small array. Known portable instruments are generally confocal and only very weakly focused, even at the focal point.
One known technique to address this uniform focus problem includes synthetic focusing, such as Total Focus Method (TFM). In this approach, typically each array element is used to transmit separately with the entire array being used for receive, and image points are formed as a linear combination of the measured data, in such a way as to produce an image that is focused in both transmit and receive at every pixel. However, TFM generates a large amount of received ultrasound measurement data and subsequently requires a long image reconstruction time. Further, TFM may not penetrate deep into the object under test, since ultrasound transmissions are made with a single element. Another known technique, known as Synthetic Transmit Aperture (STA) addresses the problem of penetration by using subarrays for formation and transmission of ultrasound beams. In contrast to TFM, STA produces an image that is receive focused at every point, and only approximately transmit focused at every point. However, because STA was intended for use in medical imaging and required immunity to source motion, STA uses several transmissions to form each of a group of A-lines, which are then scan-converted to form an image. Thus, for an array with a relatively small number of elements, STA generates even more ultrasound measurement data than TFM.
Therefore, there is a need for an improved ultrasound imaging system for non-destructive testing that produces high resolution images, at high frame rates, and has lower computational requirement in addition to lower data transport and storage requirement.